In C++, you can store 8-bit integers, 16-bit integers, 32-bit integers, and 64-bit integers:
int8_t alice;
int64_t bob;
GCC and Clang even have 128-bit integers:
__int128 carol;
You can store 128-bit integers, but you aren't allowed to write 128-bit integers:
__int128 dave = 18446744073709551616;
error: integer literal is too large to be represented in any integer type
__int128 dave = 18446744073709551616;
^
This library allows you to write 128-bit integers:
#include "suffix128.hpp"
__int128 emily = 18446744073709551616_128;
All you have to do is #include "suffix128.hpp" and end your integer with the suffix _128.
You can use the suffix _u128 for unsigned integers:
unsigned __int128 frank = 123456789012345678901234567890_u128;
128-bit integers you write this way are constants and they can be used as template parameters or enumerator initializers.
enum Bar { BAZ = 0_128 };
You can use C++14's single-quote digit separator:
__int128 gloria = 1'2'3'456'7_128;
You can also write them in hexadecimal or octal notation:
__int128 hector = 0xfeed'bad'beef'2'dad_128;
__int128 imelda = 0644_128;
You can even write 128-bit binary literals:
unsigned __int128 jules = 0b00000001000001100001010000111000100100010110001101000111100100110010101001011100110110011101001111101010110101111011011101111111_u128;